A recent and developing trend in zoo design is the use of trails for animals--expanding the area available for exploration and exercise beyond the typical exhibit footprint. I believe this trend began with the invention of the 'O-line' at the National Zoo nearly 20 years ago. The O-line provided a system of transport for the Orangutans between exhibits using their natural behavior of brachiation, and maximized efficiency by utilizing vertical, rather than horizontal, space. However, over the years, the O-line has supposedly become less utilized by the Zoo due to staffing constraints. Rumor has it that because the O-line crossed above visitor pathways, keepers were posted below the O-line for safety and clean-up when in use.
Recently, the Indianapolis Zoo released plans for an architecturally impressive International Orangutan Center featuring an updated O-line. It appears this system limits the animals' range over 'people space' and hopefully alleviates some of the concerns of the original National Zoo system.